Field of the Invention
This invention relates to control systems that have a controlled device which is actuated in response to displacements of a manipulatable control unit and wherein the control unit is subjected to a bias that opposes its displacement to provide a simulated feedback or "feel" that facilitates precise control; and the invention is more particularly concerned with a method and apparatus for varying the bias upon the control unit in accordance with predetermined criteria, to provide a "feel" that facilitates both maneuvering and trimming manipulations of the control unit.
The system for pitch control of an aircraft can be taken as typical of the various types of control systems that involve the problems and requirements addressed by this invention.
In lighter aircraft, wherein the stick or manipulatable control unit is directly linked to the related control surfaces, the biasing forces on the stick are generated by aerodynamic forces on the actuated surfaces to which it is linked, and the pilot thus becomes accustomed to interpreting stick forces as having a relationship to the forces that the control surfaces exert for changing the attitude of the aircraft. Thus the stick biasing forces constitute a feedback that assists the pilot in controlling the aircraft.
In high performance aircraft, where the stick is not directly linked to the control surfaces, a biasing force is artificially imposed upon the stick or manipulatable control unit, to yieldingly oppose its displacement away from the neutral position and yieldingly urge it back toward that position. This force gives the pilot information about the amount of displacement that he is imparting to the stick, in effect synthesizing the natural feedback or "feel" of a directly connected control system.
Conventionally, the force thus imposed upon the stick increases with increasing displacement of the stick from its neutral or null position, requiring a relatively high muscular effort for large stick deflections. Biasing the stick with a force having a gradually increasing gradient is satisfactory for large, coarse stick displacements, intended to bring about rapid change of aircraft attitude in maneuvering flight, but it has a negative influence upon fine control, accomplished with small displacements of the stick whereby the aircraft is trimmed for small changes in its attitude. Such trimming often takes place with small displacements of the stick near its null position, as in adjusting the level flight attitude, but it can occur in other parts of the range of control stick displacement, as when the attitude of a diving or steeply climbing fighter airplane is adjusted for gun aiming. Upon a reversal of the direction of stick displacement such as characterizes a trimming manipulation, there should be no change in the force that biases the stick toward its neutral position at the beginning of the reversing movement, but during that movement it is desirable that there be a high information-transmitting gradient of stick force, so that a small displacement of the stick is attended by relatively large change in biasing force upon it.
With a conventional manually controllable trim system it is possible to obtain a rather accurately controlled small change in aircraft attitude, even with large stick deflections and load factors. However, the employment of the trim system at large stick displacements has marked disadvantages that are not acceptable for high performance aircraft. The trim system is controlled by a manually operable nose-up/nose-down reversing switch that requires special attention and manipulation. It tends to respond somewhat slowly. If the trim system is actuated during a maneuver involving large displacement of the control stick, then upon return of the stick to its neutral position the aircraft will not return to the same flight attitude that existed before trimming.
Although the above described inconveniences are especially troublesome in control systems for high performance fighter aircraft, where quick and reliable control commands and reactions are demanded by the pilot, similar inconveniences are also presented by control systems in other applications.